1. Field of the Invention
The present invention relates to electric lamps, and in particular vehicle headlamps. Still more particular, the invention relates to headlamps having compound optical elements.
2. Description of the Background Art
Headlamps are designed to accomplish several goals at once. They must illuminate both near and far regions in front of a driver, without detrimentally effecting the vision of other drivers. This is accomplished at a minimum by forming a beam pattern that complies with automotive lighting requirements. At the same time, styling, aerodynamics, size, weight and cost are factors that must also be dealt with. Beam patterns are then constructed with variety of considerations at once. The beam pattern includes a region of high intensity called a hot spot that is normally built by effectively overlaying numerous reflected images from the light source. Reflectors with relatively long focal lengths, have small source images that can be grouped in an angularly narrow region to form the hot spot. At the same time, a headlamp high beam for example, must spread some light right, left, above and below the hot spot to broaden the driver's view. Reflectors with short focal lengths, have large source images that can be spread over a broad area. The conflict between short and long focal lengths is apparent. Further, headlamps should efficiently use the available light, so the source may be designed for longevity, or energy efficiency. Lamp efficiency is achieved by intercepting and reflecting a greater portion of the light from around the light source. Capturing more of the light by reflecting it from more of the surrounding spherical area, means the light is necessarily captured at a greater variety of angles. It also means relatively less spherical area is available to direct the light through to the field to be illuminated. All these factors complicate the design.
In a typical prior art sealed beam headlamp with a parabolic reflector and refractive cover lens, the light source is disposed near the focus of the reflector, so rays emitted from the light source are reflected forward, parallel to the axis of the paraboloid. The parallel beams are then refracted by the prisms and lenses of the cover lens to form a predetermined beam pattern. The design relies on a relatively large focal length to form the necessary hot spot in the beam, while beam spread is achieved by the lens optics. For efficiency, a relatively large reflector area is used to gain the necessary solid angle. The design is not particularly adaptable to fit with styling variations in the surrounding vehicle body. The reduction of the overall height for styling, and inclination of the lens surface for aerodynamics cause a significant reduction in the overall headlamp efficiency. The reduced height can, to a degree, be offset by increased width, but only with diminishing returns. Usually the total frontal area is increased in this trade off, and the large frontal area is of itself a styling and aerodynamic detriment. It is then not practical to make an efficient, parabolic reflector type headlamp with a small frontal area.
Currently, there is a trend to move the beam forming optics from the cover lens to the reflector. The headlamp then has a reflector with a complex surface, such as a compound-curvature or multifaceted surface, and a clear cover lens. Since, the clear cover lens has little or no optical effect on the beam pattern, it can be configured to carry all the styling and aerodynamic constraints. The problems with focal length tradeoffs and the degree of enclosure are approximately the same in both the parabolic reflector/refractive lens, and the complex/clear lens type headlamps. The later then still require a relatively large frontal area.
To increase efficient use of the light from the filament and at the same time allow for a small frontal area, one method is to use a projector type lamp. FIG. 1 shows a schematic side view of a projector type headlamp. These headlamps use an elliptical reflector to intercept a large portion of the light from around the light source. The large amount of collected light is then directed to a converging lens that collimates and spreads the available light. The light source is placed to coincide with one focal point of the elliptical reflector to thereby project light through a narrow region approximately at a second focal point. A mask is usually placed in the vicinity of the second focal point to block light and thereby helps define some of the beam pattern edges (cut off). The mask removes available light from being usefully projected. The light is then passed through a small reflector opening to concentrate the flux on the converging lens. The image of the filament produced by the elliptical reflector is then located at the second focal point, coinciding with the first focal point of the positive converging lens (between the reflector and lens). The rays from the filament image are then refracted by the converging lens to form the beam pattern. An optically clear cover lens may be placed in front of the converging lens for styling and aerodynamics.
A typical projector headlamp design requires a relatively long axial dimension to span the distance between the two focal points and include the reflector behind the one focal point and the lens in front of the other. The headlamp then extends deep under the hood and competes for valuable internal space. There is then a need for a headlamp forming a beam pattern including hot spot, and spread regions wherein the headlamp has a relatively small frontal area, and a relatively short axial extension.